An aqueous solution of tetramethylammonium hydroxide (TMAH), one of the group of quaternary ammonium hydroxides, is currently used in large quantities as a developer of resist membranes in the manufacture of LSI's and liquid crystal displays, as a cleaning fluid for semiconductor substrates in one of the steps for the production of semiconductors, or as a raw material for tetramethylammonium silicate and it is an industrially irreplaceable compound. In particular, in the case where TMAH is used in the aforementioned applications relating to semiconductors, the requirements for the concentration of impurities in TMAH are very rigid; for example, transition metals, alkali metals, and alkaline earth metals such as Na, K, Ca, Cu, Zn, Fe, Cr, Ni, Pb, Ti, and Sn must respectively be kept below 1 ppb. For this reason, there is a demand for a method which is capable of producing an aqueous solution of high-purity TMAH on a commercial scale at low cost.
The inventors of this invention earlier proposed a method for producing TMAH which comprises synthesizing a quaternary ammonium inorganic acid salt by the reaction of a trialkylamine with a dialkyl carbonate and electrolyzing the inorganic salt in an electrolytic cell partitioned by a cation exchange membrane (refer to Patent Reference 1). This method is free from the problematical generation of halogen ions and formate ions which corrode electrodes and degrade ion exchange membranes in the course of electrolysis and is capable of producing high-purity TMAH with a reduced content of the aforementioned impurities and excellent storage stability and, additionally, in an enhanced yield.
In an electrolytic process involving electrode reactions, insoluble electrodes are generally used to avoid the consumption of the electrodes themselves. As the performance required for insoluble electrodes varies with the products and objects of electrolysis, insoluble electrodes are classified, for example, into electrodes for generation of chlorine, electrodes for generation of oxygen, and functional electrodes (electrodes coated with platinum group metals). According to the aforementioned method for producing TMAH proposed by the inventors of this invention, oxygen and carbon dioxide evolve from the anode in the electrolysis of a quaternary ammonium inorganic acid salt in an electrolytic cell partitioned by a cation exchange membrane as shown in FIG. 1. The solution of the quaternary ammonium inorganic acid salt shows a pH of approximately 8 to 10. Electrolysis of this kind has not often been observed.
Now, an electrode of a metal such as gold (Au), platinum (Pt), and silver (Ag), a graphite electrode, an electrode formed by plating an electrode base material of titanium with a platinum group metal, a lead electrode, a nickel electrode, an electrode formed by coating an electrode base material of titanium with oxides consisting mainly of an oxide of a platinum group metal, and the like are generally used in various electrolytic processes. However, the use of any of these electrodes as an anode in the production of TMAH by electrolysis in the aforementioned manner causes problems such as degradation of corrosion resistance and durability and a rise of electrolytic voltage to incur an increase in electric power consumption and an increase in production cost on a commercial scale. To be specific, when an electrode of gold (Au), platinum (Pt), or silver (Ag) or a graphite electrode is used, the surface of the electrode peels off and the voltage rises after only a few hours of electrolysis and this makes it difficult to continue the electrolysis. When an electrode formed by plating an electrode base material of titanium with a platinum group metal is used, costly metals such as Pt, Pd, and Ru dissolve out as impurities on the order of ppm after only a few hours of electrolysis. On the other hand, an electrode formed by coating an electrode base material of titanium with oxides of Ir and Ta is intended for use in an electrolytic process in which oxygen evolves from the anode during electrolysis and it is used mainly in electrolytic plating with the use of a sulfuric acid bath or the like. In consequence, when this electrode is used in the electrolysis of a quaternary ammonium inorganic acid salt where oxygen and carbon dioxide evolve from the anode, oxygen and carbon dioxide evolve simultaneously thereby causing problems such as a rise of overvoltage in electrolysis, degradation of the durability of electrode, and an increase in the electric power consumption. A lead electrode, a nickel electrode, or a graphite electrode exhibits durability and corrosion resistance to some extent as an anode in the electrolysis of organic alkaline compounds; however, when used in the kind of electrolysis proposed above by the inventors of this invention where oxygen and carbon dioxide evolve simultaneously, the electrode in question consumes itself excessively and cannot function as a commercially satisfactory electrode.
By the way, an electrode comprising an electrode base material of an electrically conductive metal, a coating of an electrode active material composed of a platinum group metal or an oxide covering the electrode base material, and an intermediate layer of a mixed oxide of an oxide of one kind or more of metals selected from Ti and Sn and an oxide of one kind or more of metals selected from Ta and Nb disposed between the electrode base material and the electrode active material is proposed (refer to Patent Reference 2). Another electrode whose intermediate layer comprises a first intermediate layer of a compound of a rare earth metal and a second intermediate layer of a base metal or an oxide thereof is proposed (refer to Patent Reference 3). However, both of these electrodes are intended for use in an electrolytic process in which oxygen and carbon dioxide evolve from the anode during electrolysis. Therefore, their use in the electrolysis of a quaternary ammonium inorganic acid salt in which oxygen and carbon dioxide evolve from the anode causes technical and economic problems such as a rise of overvoltage during electrolysis, degradation of durability and corrosion resistance, and an increase in electric power consumption.    Patent Reference 1: JP63-15355 B    Patent Reference 2: JP59-38394 A    Patent Reference 3: JP2-5830 B